A Sensitive Reaction

A Sensitive Reaction Global warming could speed up decomposition, but how much might decomposition speed up global warming? By Kerry Grens 1 "In a sense there's some kind of natural break in the system that would bring this positive feedback to a halt," says Jerry Melillo at the Marine Biological Laboratory. For example, in a 10-year study Melillo led in the Harvard Forest, the response to warming, as measured in carbon flux, jumped an average of 28% in each of the firs

Jan 1, 2008
Kerry Grens

A Sensitive Reaction

Global warming could speed up decomposition, but how much might decomposition speed up global warming?

By Kerry Grens

1 "In a sense there's some kind of natural break in the system that would bring this positive feedback to a halt," says Jerry Melillo at the Marine Biological Laboratory. For example, in a 10-year study Melillo led in the Harvard Forest, the response to warming, as measured in carbon flux, jumped an average of 28% in each of the first six years, but by the tenth year didn't respond at all to warming.2 In other words, the researchers found that, with elevated temperatures, decomposition (and therefore carbon dioxide) rises, but then returns to normal with time, breaking down the positive-feedback loop. Why?

Wallenstein suspects that microbial communities are acclimating to long-term increases in temperature, which may favor microbes with less temperature-sensitive enzymes. "In the microbial world we understand very little how communities will change, how their functional characteristics will change," says Josh Schimel at the University of California, Santa Barbara. To study acclimation over the long term, Wallenstein is pulling in proteomics to measure the relative abundances of enzymes from each organism over time. Additionally, he is examining enzymes' temperature responses, using what he calls "crude" molecular biology techniques: He mixes up a batch of soil, fluorescently labels a particular molecule of interest, and observes how quickly the microbes break it down under different conditions. "We're looking at the temperature sensitivity of some of these enzymes that degrade chitin, cellulose, and proteins in soil," says Wallenstein

Another (and perhaps simpler) explanation for why the positive-feedback loop breaks down is that, as decomposition increases, microbes utilize all the easily accessible organic matter in the first few years of warming. Melillo's hunch is that "once that relatively easy-to-decompose material is used up, the additional carbon loss [proceeds] at a slow rate."

Melillo and others have found that warming up soils at first increases the rate of decomposition, but over time it returns to normal, despite continued warming. Scientists want to know what's responsible for the declining response.

Understanding why and when this feedback loop breaks down could make a big difference to climate models. There are two to four times as much carbon in soils (though mostly locked up in permafrost) as exists in the atmosphere; that's a lot of carbon that could potentially get into the atmosphere if microbes begin releasing it. A 2000 global warming model that included the feedback loop from increased decomposition, among other sources, estimated that by the year 2100 the temperature would increase 1.5 K more than without this feedback (5.5 K compared to 4 K).

However, this model has no acclimation built in, points out Melillo, and doing so might decrease warming estimates that include the feedback loop. "It's just a very fundamental question that has not generally been answered by experimentation," he says. The soil warming experiments at Harvard Forest are now in their 17th year, and Melillo expects to publish new data early this year. The long-term effects of warming on microbial communities remain a mystery, he says. "Only time will tell."

Correction (posted January 7, 2008): When originally posted, this story had Matthew Wallenstein as Mark Wallenstein. The Scientist regrets the error.

 


1. Y. Luo et al., "Acclimatization of soil respiration to warming in tall grass prairie," Nature, 413:622-5, 2001.
2. J.M. Melillo, "Soil warming and carbon-cycle feedbacks to the climate system," Science, 298:2173-5, 2002.
3. P.M. Cox et al., "Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model," Nature, 408:184-7, 2000.